Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. In particular, throughput and reliability can be drastically improved if both the transmitter and the receiver are equipped with multiple antennas. This arrangement results in a so-called multiple-input multiple-output (MIMO) communication channel; such systems and related techniques are commonly referred to as MIMO systems and MIMO techniques.
The LTE-Advanced standard is currently under development by the 3rd-Generation Partnership Project (3GPP). A core component in LTE-Advanced is the support of MIMO antenna deployments and MIMO related techniques for both downlink (base station to mobile station) and uplink (mobile station to base station) communications. More particularly, a spatial multiplexing mode (single-user MIMO, or “SU-MIMO”) for uplink communications is being designed. SU-MIMO is intended to provide mobile stations (user equipment, or “UEs” in 3GPP terminology) with very high uplink data rates in favorable channel conditions.
SU-MIMO consists of the simultaneous transmission of multiple spatially multiplexed data streams within the same bandwidth, where each data stream is usually referred to as a “layer.” Multi-antenna techniques such as linear precoding are employed at the UE's transmitter in order to differentiate the layers in the spatial domain and to allow the recovery of the transmitted data at the receiver of the base station (known as eNodeB, or enB, in 3GPP terminology).
Another MIMO technique supported by LTE-Advanced is MU-MIMO, where multiple UEs belonging to the same cell are completely or partly co-scheduled in the same bandwidth and during the same time slots. Each UE in a MU-MIMO configuration may transmit multiple layers, thus operating in SU-MIMO mode.
To enable detection of all of the spatially-multiplexed data streams, the receiver must estimate an effective radio channel for each transmitted layer in the cell. Therefore, each UE needs to transmit a unique reference signal (RS) at least for each transmitted layer. The receiver, which is aware of which reference signal is associated to each layer, performs estimation of the associated channel by performing a channel estimation algorithm using the reference signal. The estimated channel is an “effective” channel because it reflects the mapping of the spatially multiplexed layer to multiple antennas. The estimate of the effective channel response is then employed by the receiver in the detection process.